The present invention relates to a bearing used for a pulley that contacts a timing belt for automobile engines, an auxiliary machine driving belt or the like.
Idler pulleys are installed to increase the wrapping angle of a belt, such as a timing belt driven by the engine of an automobile or an auxiliary machine driving belt, so as to impart a suitable tension to the belt. As for pulleys, there is a pulley of the type (having the so-called "hat type" outer ring) in which a pulley peripheral surface to be contacted by a belt is formed directly on the outer surface of the outer ring of a ball bearing; however, idler pulleys of the type in which a pulley body having a pulley peripheral surface and a ball bearing are fitted together are frequently used.
In this type of pulleys, when the pulley body rotates as it receives the rotating force from the belt, the outer ring of the ball bearing fitted therein rotates together with the pulley body.
In this connection, when such pulley is operated in a cold ambient, an abnormal noise (hoot sound) is sometimes produced depending upon the pulley specifications or operating conditions. This abnormal noise in a cold environment, or the so-called abnormal noise under cold ambient, does not always occur in the market; rather, it depends on atmospheric temperature, etc., and in Japan it occurs only in very limited districts (Hokkaido, etc.). Further, it occurs in a brief time (one minutes at most) subsequent to the start of the engine of an automobile, with so such sound occurring thereafter. Further, it occurs more often when a single-row deep groove ball bearing is used as such ball bearing, while the use of an angular ball bearing of a plural row type results in lesser occurrence.
The abnormal noise under cold ambient has such complicated properties and the cause of occurrence thereof, though not yet having been clarified, is supposed from the results of various experiments to be the self-excited vibration of the rolling elements. More particularly, in a cold ambient, the viscosity of the base oil in a grease increases, tending to cause unevenness in the oil films on the raceway surfaces, which unevenness, in turn, minutely periodically varies the friction coefficient between the rolling bodies and the raceway surface, thereby causing the self-excited vibration of the rolling elements. This self-excited vibration is believed to cause the pulley system to resonate, producing the axial vibration (translation) of the outer ring, leading to the occurrence of abnormal noise under cold ambient.
Conventionally, as a abnormal noise under cold ambient preventive measure, investigations have been made as to the use of a grease which is superior in low temperature properties (a grease which, even in a cold environment, forms oil films uniformly on the areas of contact between the rolling elements and the raceway surfaces of the inner and outer rings). This preventive means is intended to effectively suppress the occurrence of abnormal noise under cold ambient by enhancing the lubricating performance of grease in a cold environment, and a substantial effect can be expected. However, since the viscosity of grease decreases, there is anxiety about the lubricating performance at high temperatures, possibly leading to a lowering of durability. Pulleys used in automobiles are operated at high temperatures and high speeds, and since their durability is one of the important characteristics, a measure that possibly leads to a lowering of durability cannot be employed.
Thus, a grease serving as a abnormal noise under cold ambient preventive measure is required to have two properties: oil film stability at low temperatures and durability at high temperatures. However, it is very difficult to satisfy these two contradictory properties at the same time.
On the other hand, even if a grease is effective for abnormal noise under cold ambient preventive measure, it could, depending upon its composition, cause brittle peeling in the raceway surface or the surface of the rolling elements. This brittle peeling, unlike the ordinary peeling in the raceway surface or the surface layers of the rolling bodies caused by metal fatigue, is a peculiar phenomenon of fracture that suddenly occurs from an inner region that is considerably deep, and it is believed that vibrations due to the trend toward higher speed cause mirror wear in the rolling surfaces, which wear, in turn, causes the formation of a fresh surface, thereby providing a catalytic action to decompose the grease, attended by evolution of hydrogen, the latter entering the steel to brittle the matrix. Therefore, in selecting a grease, it is necessary to consider not only abnormal noise under cold ambient but also the composition thereof from the viewpoint of brittle peeling.
Accordingly, an object of the invention is to provide a bearing for automobile pulleys that is capable of reliably preventing brittle peeling and abnormal noise under cold ambient.